Heat the 2 tbsp of the oil in a wide, heavy pan and add the leeks. Add most of the oil from the anchovy tin and half of the anchovies, chopped very small with a scissors.
Cook very gently, uncovered for 10-15 minutes or until the leeks are quite soft and the anchovies have totally disintegrated.
Add the chopped spinach, cover the pan and wilt for 3–4 minutes.
Meanwhile, cook the fusilli according to the instructions on the pack.
When the fusilli are cooked, drain them, leaving a little of the cooking water in the bottom of the pan.
Return them to the pan, thoroughly mix in the spinach and leeks then season to taste with salt, pepper and lemon juice and toss in extra oil as you feel is needed.
Serve warm or at room temperature.